I have one Genesis caboose with decoder and another without. The latter has LEDs in place but requires, the words say, a lighting circuit to complete - all pick ups are there already. Would it be possible to wire in a simple decoder like a Digitrax DH123 with 1000 ohm resistor to complete the circuit in much the same way as you would fit one to a blue box Athearn loco? Not taken it apart to see what the wiring is to the LEDs.

Hi Mike, yes that should be possible, any cheap DCC function only decoder will work, just make sure the current to the LED's is restricted to no more than 20mA, that's what the marker light LED's are rated as, any more and you'll shorten their lifespan.
This is another decoder that might do the trick for you. http://www.dccsupplies.com/shop/product ... cts_id=772

Alternatively, you can use straight resistors from the pickups etc, but you'd have the light on all the time, which is probably not what you want.

One my cabooses has a Lenz function decoder fitted, it operates the end lights and interior lights. The other will when I get around to it.
I would run the LEDs at a far lower current, 20mA will be very bright! 10mA will probably be easily enough, I run headlights at 15mA or lower.

You take the supply voltage, say 12v, subtract the LED forward voltage drop, about 1.2v for red, green and yellow, 3.5v for white and blue.
So for your red LED 12 - 1.2 = 10.8v.
Now to get the required resistor divide the voltage result by the required current say 10mA.
So 10.8v / 0.01A (10mA) = 1080 ohms. 1080 is not a standard resistor the closest is 1000 (1K0) which will make the LED slightly brighter or 1200 (1K2) which will make it dimmer. I normally use 1K2 for red LEDs, 1K0 for green and 680 for white.

The last thing to work out is the power dissipation required for the resistor. For that multiply the voltage across the resistor 10.8 by the current 0.01 = 0.108. So the resistor has to be rated at least 0.11 watts, 0.125 are available. Larger is ok just takes up more space, and will run cooler. I normally use 0.25 watt ones.

Each LED should have its own resistor.

There are other options such as putting LEDs in series.
This calculation is for a d.c. supply, a.c. will be different and require a diode to protect the LED.